Improve auto-fix and add formatting using Prettier compatible with ESLint config

[tiagostutz]

Before starting the actual implementation of the issue #4 I'm submitting this PR that has some formatting automatizations that would help with code review. Basically it adds:

• add prettier with compatibility config
• add prettier execution to make lint command, so the code is fixed and formatted.
• explicitly single quote requirement to eslint config
• make lint-fix target to fix all code via command-line, if needed
• settings for VSCode to auto-format using lint
• files fixed/auto-formated based on eslint rules (specially single quote)
• some CONTRIBUTING.md additions

I put single quote because I couldn't find any place where it was explicitly set and looks like the single quote